According to Timatic:
Visa required, except for Holders of onward tickets transiting Seoul Incheon (ICN), for a max. transit For details time of 24 hours.
So as long as your transit is under 24 hours and you’re transiting through Seoul you don’t need a visa regardless of your nationality or destination.
Airline agents go by Timatic, which states:
Passengers with a visa issued by Australia, Canada, New Zealand or USA to nationals of any country except for China (People’s Rep.), Cuba, Iran, Macedonia (FYROM), Sudan and Syria, only if in transit through Korea (Rep.): -holding confirmed onward tickets on flights departing within 30 days; when arriving from a third country, departing to the country that issued the visa (e.g. DEL-ICN-YVR); or arriving on a direct flight from the country that issued the visa (a visa that expired on departure from that country is accepted), departing to a third country (e.g. YVR-ICN-DEL).
In other words: yes, you can enter Seoul and spend one night there. However, print this site to show the Airline agent just in case
Visa-free Entry for tour and transit
Nationals of the countries below can enter Korea without a Korean visa for a stay of up to 30 days, if their purpose of visit is tour or transit
MACAO, BRUNEI, SAUDI ARABIA, U.A.E, YEMEN, OMAN, JAPAN, QATAR, TAIWAN, H.K(90days), KUWAIT
※ Only Diplomatic·Official passport : INDONESIA, LEBANON
GUYANA, GUATEMALA, VENEZUELA, ARGENTINA, ECUADOR, HONDURAS, URUGUAY, PARAGUAY
MONACO, Vatican, BOSNIA HERCEGOVINA, Cyprus, SAN MARINO, Serbia & Montenegro, SLOVENIA(90days), ANDORRA, ALBANIA, CROATIA
GUAM, NAURU, NEW CALEDONIA New Caledonia, MICRONESIA, SAMOA, SOLOMON IS, KIRIBATI, FIJI, AUSTRALIA(90days), MARSHALL IS, PALAU
SOUTH AFRICA, Mauritius, Seychelles, SWAZILAND, EGYPT
(Note that Vietnam is not on the list.)
Visitors meeting the following requirements also can enter Korea without a Korean visa for a stay of up to 30 days, if their purpose of visit is transit
Note) Nationals of Cuba,Macedonia ,however, must apply for korean visa, even if they meet these requirements
Holding a visa or a re-entry permit of the U.S. Japan, Canada, Australia, or New Zealand (5 countries)
Travelling from either of the five countries to a third country via Korea, or travelling from a third country to either of the five countries via Korea
Holding a confirmed onward flight ticket for departure within 30 days
No Immigration offence records, while in the five countries
Those who have visited Korea more than 4 times in recent two years, or those who have visited Korea more than 10 times in total can enter Korea without a Korean visa, notwithstanding the requirements above
Note) However, this facility for frequent visitors is not applicable to nationals of “China (P.R.C)”, “Cuba” , “Macedonia”
It appears that you are eligible for transit without visa if you have no past immigration offences in any of the five countries.
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